This invention relates to a coating for phosphors, specifically to a phosphor coating of polyvinyl alcohol, and to such a coating modified with amino acids, and also to articles of manufacture employing the coated phosphors.
Two general methods of preparation of tridot color cathode ray tube screens for color television are wet screening and dusting. In wet screening, slurries of each phosphor (red, blue and green) are mixed separately in solutions of polyvinyl alcohol rendered photosensitive by the addition of a hexavalent chromium salt, for example, ammonium dichromate. Each slurry is uniformly spread on the faceplate of the cathode ray tube by rotation of the faceplate. The layer thus formed is exposed with a pattern of light corresponding to the pattern of dots desired to be formed on the screen. The exposed area becomes insoluble in a "developer," such as water. Therefore, when the screen is washed or "developed," the exposed area remains while the unexposed area is washed away, leaving the desired dot pattern. The procedure is repeated for each color to obtain an array of red, blue and green phosphor dots on the screen. Such dots are usually circular in shape, but could be oblong, or any other desired shape.
In the dusting method, dots of photosensitive material without the phosphors are made by exposing and developing as described above for the wet method. Then the phosphors are deposited on the wet or sticky dots by "dusting."
During preparation of the screen by either method, a number of requirements must be met by the phosphor. The phosphor must (1) be wettable by the photosensitive material, (2) not react with the photosensitive material, (3) form a good adhesive bond with the photosensitive material, and (4) wash out of the unexposed areas. In the dusting procedure, there is the added requirement that the phosphor must flow freely, in order to avoid plugging of the dusting apparatus.
Most commercial phosphors, especially the red, blue and green required for tridot color cathode ray tubes, will not meet the above requirements. In the past, such phosphors have been coated with some inert material in order to modify the surface characteristics of the phosphor to make them usable in the screening process, such as silica and pyrophosphates. Sr.sub.2 P.sub.2 O.sub.7 has been used to improve adherence on the faceplate and to eliminate "haze," the non-purity of color television pictures resulting from either incomplete washing out from the unexposed areas, or cross-contamination of two or more of the phosphors, or both.
An alternate coating for the blue ZnS:Ag phosphor, magnesium aluminum pyrophosphate and silica, is claimed in allowed U.S. patent application Ser. No. 729,905, filed Oct. 6, 1976 now U.S. Pat. No. 4,103,069, and assigned to the present assignee.
However, there is a need for a single coating system which is both compatible with the commonly used dichromate sensitized polyvinyl alcohol photosensitive material and all of the phosphors used in tridot color cathode ray tube screen manufacture. Accordingly, it is felt that such a coating having universal compatibility would be an advance in the art.